BOILER INSPECTION GUIDELINE Regardless of the original equipment manufacturer, NEE Process Solutions can offer engineering know how not only to improve unit performance and reliability but also to eliminate operating and maintenance problems Boilers of the PC or CFB type.
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BOILER INSPECTION GUIDELINE
Regardless of the original equipment manufacturer, NEE Process Solutions can offer engineering
know how not only to improve unit performance and reliability but also to eliminate operating and
maintenance problems Boilers of the PC or CFB type.
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PURPOSE OF BOILER INSPECTION
The purpose of a Boiler Inspection is to:
Use planned outage time effectively to ensure that unit availability, safe operation and
equipment life is maintained.
Reduce forced outages due to maintenance failures.
Effectively plan outage-required preventative maintenance activities and periodic
replacement of normal wear parts.
A typical boiler inspection will deal with the following component areas:
Waterside
Fireside
Boiler Externals
Fuel Firing Equipment
Air/Flue Gas Systems
Auxiliary Equipment
Controls (if necessary)
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OUTAGE OBJECTIVES
Major scheduled outage should be structured around customer objectives:
Perform known maintenance tasks.
These items are scheduled based on historical data, past outage inspections or items noted during
the pre-outage operational unit walk down.
Inspect equipment to identify areas needing repairs.
Certain equipment can only be inspected during non-operational periods.
Perform preventive maintenance tasks.
Scheduled or routine maintenance includes such items as turbine bearing inspections, hydrostatic
testing of pressure parts, checking and documenting tube minimum wall thickness, packing valves,
etc.
Upgrade equipment and make design changes as part of a plant betterment program.
Implement state-of-the-art improvements to enhance unit operation and eliminate generic design
problems identified by the manufacturer.
Establish a maintenance history for future use.
This objective is perhaps the most important. It helps reach the goal of all the proceeding objectives
and is required if an ongoing comprehensive maintenance program is to be effective. A maintenance
history is developed by thoroughly documenting the outage and inspection findings for every
scheduled outage.
PERFORMING THE INSPECTION
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The boiler inspection identifies and evaluates problem areas (current and potential). This allows the
service engineer to recommend repairs and solutions. The problem areas will have varying levels of
priority.
During an outage, every portion of the unit is thoroughly inspected. The inspection is divided into
three general categories:
The pre-outage walkdown, in which the entire unit and its subsystems are inspected under
operating condition.
The internal inspection of the unit after shutdown and its subsystems
The post-outage start-up inspection in which all the equipment is checked as it is returned to
service.
Inspection activities include visual observations, comparisons, measurements and non-destructive
examination techniques. Accurate record keeping of findings as well as careful labeling of items in
the field will make it easier to communicate punch list items and write the final report. Photographs
are another method to capture and express details.
PRE-OUTAGE WALKDOWN
The pre-outage operational unit walkdown is a significant first-step of the unit inspection. While the
unit is on line, the inspector can assess the operating conditions of the unit and note any
discrepancies that require attention during the outage. Several problem areas are more apparent or
best observed while the unit is still in operation, i.e. safety valve leakage, expansion trams, load-
spring hangers, insulation leakage, etc.
The walkdown should cover the complete unit from top to bottom and all the auxiliary systems.
Auxiliary systems are noted here to demonstrate the scope of a complete unit walkdown.
Structure Components
Boiler Support
Inspect all boiler hanger rods for integrity. Inspect variable load and constant load spring hangers
for loading indications. Note any bottomed-out spring hangers. Also, note any loose hanger rods.
Check all vertical and horizontal buckstays for warpage or misalignment. Inspect all buckstay
stirrups, bolts, nuts, and washers for integrity. Check expansion trams for alignment and note
readings at all reference points with the boiler hot.
Structural Steel
Inspect the structural steel for any interference with the boiler or auxiliary equipment. Inspect the
grating and walkways for missing or loose sections. Check handrails for any missing or broken
sections. Make note of all discrepancies.
Insulation and Lagging
During walkdown, inspect for any areas of missing insulation. Check for discoloration of the
insulation, which would indicate leakage of hot gases or air.
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Check duct insulation in areas of expansion joints for signs of buckling. Ensure the presence of
insulation in areas around inspection ports, sootblowers, and access doors. If possible, check areas
of poured refractory for damage. Note all problem areas observed.
Upper Level Components
Safety Valves
Inspect for leakage around the stem or packing. Note if the valve is leaking across the seat so that
some amount of vapor is discharging from the vent piping. Check for pluggage in the drip pan.
Check for binding or interference between the safety valve and vent piping. Make note of abnormal
conditions.
Sootblowers and Furnace Probe
Check for local and remote operation by cycling each sootblower through its operating sequence.
Check for steam leakage in the sootblower supply lines, valves, and swivel tubes. Check the
sootblower wall box for damage. Check the drive mechanism on all sootblowers. Make sure that
cranks and tools are available to retract a sootblower that has stopped in the "advanced" position.
Observe movement of air heater sootblower swivel mechanism. The furnace temperature probe is
usually made by the same manufacturer as the sootblowers and should also be inspected at this
time.
Fuel Components
Ignitors
Operator to remove operating ignitors from service in order to check the indicator lamps on the local
ignitor control station. Have operator place each ignitor in service to verify operation and to check
the indicator lamps. Inspect electrical cables, oil, gas, and air supply lines.
Tilting Tangential Firing System
During walkdown, check the nozzle tilt indicators for degree of tilt indication of the fuel and air
nozzles. Check all overfire air nozzles for degree of tilt. All corners of tilt indicators should be within
5° of each other.
While in this area, inspect all coal piping entering the windbox for wear, damage, or leakage at
elbows or couplings. Also check all fuel pipe hangers. Note the position of all windbox air dampers
as indicated by the scribe mark on the damper shaft. All dampers on the same elevation should be
at the same position. Inspect the windbox and related duct work for leakage.
Oil Guns
The oil guns are located with the fuel and air nozzles in the corners of the furnace. Check all oil and
steam or air lines for leakage; check all related piping and valves; inspect the oil gun advance and
retract mechanism; and check that spare oil guns are properly cleaned and stored.
Coal Piping
Inspect the coal pipes for indications of wear. Check the coal pipe couplings for signs of leakage. If
so equipped, check the coal pipe constant load spring hangers. Examine for any coal pipe related
expansion problems.
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Coal Feeders
During the inspection of an operating gravimetric feeder, check the position of the tension pulley,
the feeder belt tracking, and the integrity of the feeder housing. Broken observation windows or
improperly closed doors negate this safety feature of the feeder.
On a C-E volumetric feeder, inspect the drive clutch assembly, the movement of the hinged leveling
gate lever, and the feeder housing. Check the drive motor and gear reducer for unusual noises and
verify proper lubrication levels.
Pulverizers
Inspect the mill foundation for cracks. Check the gear case for proper oil level, temperature, and
signs of leakage. Examine the material being rejected from the mill; excessive coal discharge could
indicate worn or improperly adjusted mill internals. Verify the movement of the three journal
assemblies for uniformity. Inspect the separator body for signs of coal leakage. Make a notation of
the classifier settings; they should all be the same. Inspect the mill motor, filters and foundation.
On exhauster type mills, check the exhauster casing, foundation, and bearing assembly. Check for
excessive noise or vibration from the pulverizer gear housing and exhauster bearing housing.
On pressurized pulverizers, check the gear case and journal seal air systems for leakage or crimped
lines.
Fuel Handling Systems
Inspect all coal handling systems. Note any excessive spillage or accumulation, and check all oil and
gas piping for leakage.
CFB Boiler Limestone Feed System
Inspect the limestone feed system for erosion, loose mounting hardware, proper clearances and
binding of rotating equipment. Note any excessive spillage or accumulation, and check all oil and gas
piping for leakage.
Auxiliary System Components
Air Preheaters
Inspect the upper and lower bearing assemblies. Check oil level and for oil leakage. Listen for any
loud noise, which might indicate problems with the air preheater seals. Check the drive motor and
gear reducer for signs of oil leakage, and verify the operation of the air preheater sootblowers.
Fans/Air and Gas Ducts
Check for the following: foundation cracks or loose anchor bolts, vibration meters for excessive fan
or motor vibration, motor amp readings, and bearing temperatures. Inspect fan housings for
damage and check all air and gas ducts for leakage, expansion problems, and missing insulation.
Ash Removal Systems
Inspect all piping for leakage and pluggage.
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Boiler Water Circulating Pumps
Inspect motor and related piping for leakage. Check the pump suction and discharge pressures.
Check and record motor cooling water temperatures. Compare data to normal operating conditions.
Note abnormal conditions.
Pre-boiler Systems
Check all components and piping for leakage. Check all components for any missing insulation.
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INTERNAL INSPECTION – WATER SIDE PRESSURE PARTS
The internal inspection requires unit downtime. Lots of maintenance activity is also scheduled
during the outage. The internal inspection activity is complex and covers a large work area.
This section covers only the water-side pressure parts of an inspection plan.
Steam Drum and Internals
Examine the steam/water separating equipment. Inspect the turbo separators, both the primary
and secondary stages. Look for corrosion, deposits, erosion, missing parts, etc. Examine the
condition of the corrugated plate dryers and the return piping.
During inspection:
Check the condition of the seal around the manway door.
Check the area around the inside of the manway door.
Check the interior of the drum for corrosion and deposits.
Check the condition and mounting of the chemical feed pipe, the blowdown pipe, and the
feedwater distribution header.
Check the downcomer nozzles, screens, and vortex eliminators.
Check all drum internals for wear and fit.
Thoroughly examine the drum liners for cracks. Cracks in the liner will allow boiler water to bypass
the steam separation equipment and allow the carryover of suspended solids into the superheater.
Lower Waterwall Drums
During inspection, crawl through the drum checking for cracks or crack-like indications particularly
around nozzle welds and manway access doors.
Generally, these cracks are shallow, not much deeper than 1/32". Usually found on the wetted
surfaces of the drum is a corrosion indication, which could play a part in promoting a crack
penetration.
If cracks or crack-like indications are found, determine the depth by a number of spot grindings.
Information on the depth together with the specific location of the cracks will permit calculations to
be made to determine whether the remaining material thickness is sufficient to meet the
requirements of the ASME Boiler Code for New Boiler Design and Construction.
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Potential Cracking Sites
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During inspection, check the screens or strainers located in the drum. These screens prevent foreign
material from plugging the orifices. The screens must be intact and firmly secured in place. If any
large holes have developed in the screens, recommend that they be replaced.
Typical Lower Drum Arrangement
Check the locating pin, orifice clamp, and fastener for deterioration, fit, and tightness. Check for
gaps between the orifice adapter and header counter bore. If gaps are found, the adapters may
need to be replaced, repositioned, or repaired.
Check the diameter of the furnace wall supply tube orifices using "Go/No Go" gages. If an orifice is
found to be worn, that is, the opening is enlarged, recommendation should include replacement.
The orifice plates assure that each tube circuit gets an adequate flow depending on its location in
the furnace. Orifices are numbered for reference and have indexing holes so they cannot be
incorrectly placed. Orifices could be either fouled or plugged with deposits, or enlarged by wear or
corrosion. If the orifice is fouled, that is, the opening restricted, recommendations include cleaning
during the outage (if possible) or replacing. Check the orifice mounting and locating pins, and the
clamp and fasteners for signs of deterioration and looseness. Inspect the orifice adapter for signs of
bypassing flow. If problems are found, recommendations might include replace, reposition, or repair
the adapter.
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Headers
To perform an internal examination, remove the handhole inspection ports on the header. Examine
the interior for corrosion, deposits, or any other foreign material. Check the area around the
handhole for any signs of cracking. Check the handhole port seal.
During an external examination, visually check the entire header for corrosion, erosion, etc. Visually
check the header nipple welds for signs of cracking. Note cracking and make recommendations for
repair.
Header Inspection
If the header is insulated or covered with refractory, note the condition.
Also inspect all other welds, especially for different material welds (DMW), near the header. If any
cracking is found, determine depth and location, and consult Engineering for recommended repairs.
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Inspect the area around each header for signs of potential problems. Inspect the area where a
header penetrates a wall or floor for cracks or expansion problems.
Check all hanger rods for tightness. Also check condition of hanger rods, clevises, and clevis pins for
bowing, overheating, and damage. Hanger rods are made of a tempered material and should not be
cut and rewelded. If significantly damaged recommendation should include replacement.
The combined circulation units are supplied with orificed waterwall inlet headers that can be
internally inspected if the handhole caps are removed. These headers should be checked for
cracking, deposits (especially on the orifices), loose marmon clamps, and cracking between the
internal partition plates and the ID of the header. Header cleanliness is a must.
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INTERNAL FIRE-SIDE FURNACE INSPECTION
The second part of the internal inspection is the fire-side (gas-side) or furnace inspection. Again, this
is a large area to inspect, with complex components. This section is divided into component
sections.
Bottom Hopper Area – PC Boiler
The coutant sloped bottom tubes and the bottom ash hopper comprise the lower furnace area.
Inspect casing, plates, screens, and the structural condition of expansion and support members.
Remove all debris, bottom ash, and water remaining in the hopper enclosures. Examine the water
seal trough material and structural condition. Inspect trough for corroded and deteriorated lining
and structural support. Look closely at all welds for signs of cracking and indication of expansion
problems.
Examine the seal plate for indications of corrosion, deterioration, and cracks in the surface.
Inspect splash screen for tears and holes in screen material. Examine all support structures for
indication of expansion problems. Inspect for signs of corrosion and deteriorating conditions.
Inspect drip shield for signs of erosion or corrosion.
The attachment welds securing the drip shields to the waterwall tubes should be inspected and dye-
penetrant checked, if cracks are suspected, as some tube leaks have been experienced at these
welds.
Inspect the slope tubes for signs of erosion, corrosion or thermal stress. Problems in this area could
be caused by sliding ash and slag, chemical reaction of the ash and moisture, splashing or surging,
wave propagation, and flooding during filling. Some possible solutions are to maintain ash hopper
normal water level at least 30" below the tubes and have an overflow capable of handling excessive
amounts.
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Furnace Seal Inspection
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Lower Combustor – CFB Boiler
Maintenance Issues for the Lower Combustor / Hearth Zone are directly related to Unit Operation.
Improper SA Flow or drop in pressure can result in overheating of the Lower SA Ducts and Start Up Burners.
Improper PA Flow
o Low flow can cause the Fluid Bed to Slump and ash to enter PA Plenum.
o High flow can contribute to accelerated erosion of PA nozzles.
Start Up curve for refractory cure can reduce some refractory spalling repairs.
Cracks in the Plenum Corners above the header and at sidewall gusset supports due to unit expansion.
Inspection Focal Points
PA Nozzles –ash pluggage, broken, and erosion
PA Plenum –ash build up, gusset supports, corners
FBAC ACV Inlets –Grease Air Ports
SA Ducts –Overheat damage
Fuel Chutes –Ceramic Tiles
Start Up Burners –Erosion, overheating
Ash Return Vents –Gaps
SRD Duct Outlets –Refractory and Grease Air
FBHE Return Ducts –Refractory and FA Nozzles
Refractory –Spalling of ledges
Water Walls for erosion and condition of AMSTAR Spray
Lower Dead Air Space
Inspection includes the examination of all skin casing, insulation, refractory, seal boxes, tube
assemblies, and the structural condition of all expansion and support members, supports, braces,
and hanger rods.
Examine all support structures for broken welds, disconnected, broken, or bent rods and missing
nuts and bolts.
Check for bent or twisted supports.
Check all slope tube support buckstays.
Check all framing supports.
Examine condition of bottom slope tubes.
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Dead Air Space Support Steel and Hangers
Hanger Disengaged from Support
Visually inspect the dead air space for ash accumulations due to gaps at the slope tube membranes.
Also check sidewall casing structure for any indication of defects or cracks.
Damaged Waterwall Slope Tube
Large clinker or ash accumulations can build up in the tube assemblies in the top of the furnace and
eventually fall, damaging the lower slope tubes and the structural steel inside the lower dead air
space, especially near the side wall corners.
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Visually inspect the bifurcated tube membranes for cracking near the membrane end at the bottom
of the side wall tubes above the bifurcate. Dye-penetrant checking of this area is also recommended
if there is any history of tube failures. If tube failures have occurred or cracking is evident, treat the
membrane as follows:
Contour the ends of the membrane welds, creating a smooth radius. Cut the middle of the
membrane back from the end a few inches and drill a hole at the end of the cut. Use a pencil grinder
to remove any rough edges from the cut and hole.
Examine sidewall seal boxes for cracks, tears, and signs of expansion problems.
Side Waterwall Seal Box Crack
Furnace pressure or expansion can cause the slope wall scallop bars to tear adjacent to the sidewall
scallop bars. The tear can extend into the side wall tube, causing a failure.
Inspect seal plate condition where waterwall downcomers penetrate lagging. Check expansion
joints (boots) at the lower strut penetrations.
Furnace Waterwalls
Lower Sidewall Tubes
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Inspect the lower sidewalls, looking for signs of erosion caused by sliding ash. Also examine the area
around the lower sootblower openings for erosion.
On many units, the lower sidewall tubes at the center point are tied together by a solid plate.
Failures have occurred in this area, along the tube membrane, due to fatigue resulting from
concentrated areas of thermal stress or corrosion attack, at the high stress concentrations. Inspect
this area, looking for signs of fatigue or corrosion. If damage is found, replace the first two or three
tubes to either side of the centerline and implement the lower sidewall modification.
Furnace Bottom Slope Tubes
Inspect waterwall tubes in the coutant bottom for gouges, dents, bowing, and overall damage due to
slag falls, slag erosion, or a combination of the two.
Erosion (abrasion wear) from tumbling and sliding slag or flyash can wear down the tube surface
exposed to the furnace area. When this happens, the tube can fail from excessive thinning or from a
minor slag fall rupturing a weakened tube wall.
Large sections of slag from the upper furnace can dent the tubes and rupture them at the impact
area or cause an overheating failure elsewhere due to restricted flow.
Failure can occur in the dead air space side of the tubes, at the steel support lugs. The great force of
slag falls can deflect the hopper slope tubes along with the support steel.
Inspect for leaks in the sealed membrane. Leaks can result in overheating of the structural support
steel in the dead air space.
GUIDELINE
When evaluating gouges and dents in slope tubing, indentations that reduce the wall thickness
below the original Minimum Wall Thickness (MWT) should be considered for repair.
Tubing should be considered for replacement when wear or corrosion has reduced wall thickness
below original MWT.
Individual tubes or tubing panels should be considered for repair or replacement when warpage or
bowing has deflected the tube(s) more than 1 tube diameter out of line.
Waterwalls
Thoroughly inspect all waterwalls. Give particular attention to the following
Waterwalls around sootblower openings
Waterwalls around the windbox openings
Waterwall corners
Waterwalls in the high heat absorbing areas
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The extended side walls
Examine waterwalls around sootblower openings for erosion and wear.
Inspect the tubes adjacent to the blowers frequently to determine tube wall thickness. Ultrasonic
testing equipment and actual measurements can be helpful in determining any tube wastage.
If erosion is noted, check the alignment of the blower. The centerline of the swivel tube should be
perpendicular to the face of the boiler tubes. Check the distance of the centerline of the cleaning
nozzle to the face of the boiler tubes.
Inspect the waterwall tube membrane at the sootblower opening. In some instances, this
membrane can be relieved with a saw cut through the center.
Examine the waterwalls in the vicinity of the firing zone for corrosion and wastage. Thoroughly
check the walls around the windbox and corners.
Adjust coal fineness to prevent coarse coal from reaching the furnace.
Maintain equal distribution of coal to all fuel nozzles. Verify by clean airflow distribution testing of
the pulverizers and/or coal line mass flow tests.
Centerwall
On divided furnace units with centerwalls, observe and record the amount and direction of
centerwall tube panel bowing. While some panel bowing is acceptable, bowing in excess of several
feet out-of-plane may be indicative of operational or structural problems and should be investigated
further.
Deflection Arch
Rear waterwall tubes form the deflection arch, projecting forward at the top of the furnace rear wall
and then sloping back underneath the superheater and reheater vertical spaced assemblies.
Examine the upper arch for signs of erosion. Flyash/sootblower erosion can cause considerable
damage to the tubes and peg fins.
Closely inspect arch tubes adjacent to the furnace centerline for evidence of sootblower erosion.
Erosion is more prevalent in this area due to the droop of the lance as it is extended into the
furnace.
Check the entire arch. Depending on unit operation, erosion can occur either on the nose or 3 to 4"
above the bend. Eddying of flyash above the bend can result in even wear, problems may be
difficult to detect by visual observation.
Refractory under the tubes can be eroded away exposing the skin casing, resulting in skin casing
cracking and warping. On newer units the upper arch tubes will be of solid membrane design, which
in most cases has eliminated skin casing problems. Overheating of upper dead air space support
members can also result and can affect the structural integrity of this area.
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Examine the rear waterwall hanger tubes for erosion. The backs of the rear waterwall hanger tubes
are susceptible to flyash erosion where they penetrate the upper slope of the arch. To prevent
damage and subsequent ruptures, these tubes should be shielded at least 4 to 6" up from the arch
tubes and extend shields down to the seal box. In some instances, abnormal flue gas velocities
under the pendants will erode the front side of the tubes. To prevent damage these tubes should be
shielded at least 12" up from the upper arch tubes.
Check all penetrations through the arch for indication of seal damage. Check the steam-cooled
spacer tubes for erosion where they penetrate the upper arch.
Check for extended sidewall movement. Sidewall tubes can move away from the wall by as much as
1 to 1.5" allowing flyash to work its way down and bow the casing.
Check clearances between the reheater vertical spaced assemblies and the deflection arch
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Nose Arch Inspection Points
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Location of Wear
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Superheater Assemblies
The superheater inspection includes the external examination of all tube assemblies. It also includes
the examination of all fluid, steam, and mechanical spacers, tube shields, pad welds, flexible spacers,
and band type spacers.
To inspect the superheater, erect scaffolding or sufficient sky-climbers must be provided to
effectively conduct a thorough examination of all superheater assemblies and support members.
Inspect all vertical superheater assemblies for signs of hard ash deposits. If the deposits are
significant, recommend removal at this time.
Examine each group of assemblies separately, since each group is affected differently by gas flow.
Review previous inspection reports, then determine areas of concern.
Inspect tube assemblies for any sign of failures in the area of dissimilar metal welds. Variables that
can promote this failure are the following:
High temperature
Time in service
Differential expansion
External loading
Cycling of unit
Inspect all tube assemblies for any signs of warping, bulging, and swelling, which might be
indications of overheating.
CRITERIA: If tubes are swelled 2% over specified OD, a sample should be taken for analysis.
When measuring tubes, take measurements at least 18 inches from the shop welds or bends. This
eliminates the effects of manufacturing processes on the tube dimensions.
If problems have occurred in the past, recommend removing a tube sample section and send it to
the laboratory for analysis.
Conduct wastage surveys in selected sections to assess the tube wall thickness. Document the
results to compare to measurements in future outages.
Superheater Division Panels
Steam-cooled spacer tubes help maintain the alignment of the division panels and minimize panel
sway. These spacer tubes bifurcate at the front of the furnace and proceed horizontally across the
left and right sides of the division panels. Because of this, wear between spacer tubes and the
superheater division panels can be a problem and can cause considerable damage if overlooked
during an outage.
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During the inspection, examine all elements and assemblies for any indication of bowing or signs of